1. Field of the Invention
The present invention relates in general to analog signalling and in particular, to frequency error detection methods and systems using the same.
2. Description of the Related Art
Dual Tone Multiple Frequency (DTMF) encoding is a common form of signaling used in telecommunications applications, such as touch-tone telephones. Typically, a composite signal representing given information (e.g. a letter or number) is generated by summing two audible tones of different frequencies. This composite signal is then transmitted, for example over the telephone lines, and then decoded at the receiving end to recover the original information.
Consider the typical touch-tone phone. The number buttons, along with the * and # buttons, are arranged in a rectangular array of rows and columns. Each row and each column is associated with a distinct frequency. In the conventional telecommunications scheme, the standard row frequencies are 697 Hz, 770 Hz, 852 Hz and 941 Hz and the standard column frequencies are 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz. Thus, when a given button is pressed, a composite signal is generated from the frequencies assigned to the corresponding row and column. For example, if the “1” button is pressed, the composite signal is generated from the Row 1 frequency of 697 Hz and the Column 1 frequency of 1209 Hz.
While DTMF signaling is relatively straightforward in theory, several difficulties arise in practical applications. Among other things, the frequencies and pulse width must be controlled to insure the integrity of the encoding process. Generally, the DTMF telecommunications specification requires that frequencies within 1.5% of nominal always be detected, those with 3.5% or more frequency error never be detected, and those with a frequency error in the range of 1.5 to 3.5% be interpreted as “don't care.” Similarly, pulses longer than 40 ms should always be detected, those shorter than 23 ms should never be detected, and those having a pulse width in the range of 23-40 ms should be interpreted as “don't care.” The minimum amplitude which must be detected is −36 dB.
The DTMF specification accounts for some signal degradation during transmission. There are two primary forms of degradation that must be considered during the detection process, name, Twist Factor and Tone Frequency Tolerance. The Twist Factor is essentially the difference in amplitude of the different frequencies making up the composite signal as a result of non-uniform power loss. Typically, the detector must be capable of detecting a signal. When power is high, frequency tone should be within +4 to −8 dB of the low frequency tone. Tone frequency tolerance is specified in terms of percentage of nominal frequencies. For example, a disallowed 3.5% variation at the tone frequency of 697 Hz is 24 Hz. At the same time, an allowed frequency variation of 1.5% at 1633 Hz is also 24 Hz. Hence, using an absolute tolerance (e.g. 24 Hz) as a basis for determining if a tone is within tolerance will lead to erroneous results.
DTMF has many advantages, and therefore improvements in the circuits and methods of signal detection are highly desirable. Correct tone detection with optimized time of detection overhead leads to quicker establishment of calls, which in turn improves infrastructure utilization.